You can do it any time you like, but if you do it while they're dormant, they wont green up again until spring when growth resumes, you can cut them back as far as you like, Derek.

Many thanks for that Derek, I think i,ll leave it till around February, hopefully it will be a little warmer then, but shan,t hold my breath

If you want to reduce the width drastically on both sides it is a good idea to take two years over, it to give one side time to green up again before cutting the other.

I have never bothered to drag it out over 2 years, I just get on with it, it will all green up again within a couple of months anyway, so I don't see the point of dragging it out, Derek.

I agree with Derek

Think Stera. has a good point, these hedges can look awful for quite a while until re-growth begins.They say prune in June on Derby day which is when I do mine or there abouts!

I think it depends on how drastically you want to cut it. If you cut back into pretty old wood it takes longer to green up than just a heavy prune does, and if you do one side at a time you still have privacy from the leafy side. Doesn't seem to matter what time of year you do it!